In high volume production printing, a stacking module can typically be used to assemble large stacks of printed material. These stacking modules can serve as an intermediate storage or base configuration for subsequent finishing procedures. Stacking modules are used with a wide range of media types, weights and sizes.
Certain known stacking modules utilize different methods to feed a supplied sheet to a top of the stack of sheets. Vacuum belts can be used to drive the leading edge of a sheet across the stack. The vacuum is then removed to release the sheet and allow it to drop onto the stack. Slotted disks can be used to capture the leading edge of a sheet and then flip the body of the sheet onto the stack. Pinch nips can be used which corrugate the sheet in the direction of sheet travel, allowing the sheet's leading edge to project substantially over the stack. The sheet is thus ejected from the pinch nips onto the stack. For each of the above cited known methods, operational printing rates in excess of 150 pages per minute (ppm) are difficult to achieve while maintaining high reliability. Therefore, a different sheet handling method is desired for printing systems capable of print rates exceeding 150 ppm.
As an alternative, resilient belt compiling technology is a well known method for stacking sheets at high print rates with high reliability. Exemplary resilient belt compilers are practiced by Hunkeler™ and Lasermax/Roll Systems™ cut sheet stacker modules. The resilient belt compiler escorts/feeds a single sheet at a time for deposit on top of the stacker module via a multiplicity of resilient drive members such as o-rings. Each resilient member provides a very low net drive force to the top of the sheet. The incoming sheet is captured between the resilient belts and the top of the stack. The friction coefficient of the belts is sufficiently high to provide a net driving force to the top of the incoming sheet. However, existing instantiations of the resilient belt technology do not permit mixed sheet length compiling.
By way of example, in known fixed sheet length compiling, sheets are driven across the top of the stack by the resilient belts until each sheets lead edge contacts a lead edge stop. At this point, the sheet stalls and the resilient belts slip against the top surface of the sheet in order to drive the sheet into registration on the stack. The lead edge stop is positioned such that the sheet's trail edge just passes a fixed trail edge wall before stopping. This ensures each sheet's trail edge drops below an input delivery plane so that the trail edge does not interfere with the next sheet lead edge.
A problem can occur if this system, method, or apparatus is used to feed mixed length sheets. If a relatively shorter sheet is fed into the stacking module in the known configuration, the lead edge of the shorter sheet abuts against the lead edge stop, while the trail edge is shorter than an inner length of the tray. Accordingly, a subsequent sheet entering the tray by the resilient belt feed will have a lead end thereof abut the trail end of the shorter sheet, thus causing a malfunction. For this reason, fixed sheet resilient belt compilers have been unsuitable for use with mixed length sheets, and such compiling has been unavailable to the consumer.
Current solutions to the problem include simply stacking common length sheets in the fixed length stacking module and inserting the different length sheets at a later time. This solution is unattractive since there are additional stack processing steps and thus additional opportunities for errors and sheet damage.
Thus, there is a need to overcome these and other problems of the prior art and to provide a system, method, and resulting device for mixed sheet length stacking in a resilient belt type stacker module.